Geology and Exploration of Kushikino Mine, Kaoshima Prefecture

Abstract

The Kushikino mine, one of the largest gold-silver mines in Japan, is located in the south-western Kyushu gold deposits field, Kagoshima Prefecture, Japan. More than twenty veins are found in two pyroxene andesite lava and its pyroclastics of Miocene age. Tectonic movements of this district after the Miocene age are;
1) Deformation caused by NW-SE trends compression at the early Oligocene, which have brought the Hokusatsu bending structure and several faults with a trend of NW-SE.
2) Marine transgression from the middle Oligocene to the early Pleistocene.
3) Upheaval movement in the middle Pleistocene.
As a result of failures of the country rocks that have experienced different stresses, fracturings and mineralizations were happened following the next three stages. The first stage is semi-opaque quartz intrusion forming linked veins and intense silicification to the wall rocks. Fractures of this stage are closely related to warped folding of the country rocks with the axis direction of E-W or ENE.
In the second stage, milky quartz and calcite were deposited along the repeated cracks and fissures. Deposition of gold and silver took place in this stage, which shows crustification bandings, ring shapes and brecciated structure. It is considered that these cracks and fissures have been produced by the marine transgression immediatedly after the first stage mineralization.
The last stage is faulting which shows small displacement. This is chracterized by shear-ing and minor pyritization. The upheaval movement in the middle Pleistocene may have formed these shear faults.
General trends of the first stage veins are classified into the following three groups.
strike
N60°E
N40°W
N80°W
dip
40°-50°S
75°W
80°S
(thrust plane)
(normal stress plane)
(shear plane of second order)
Because of the tendency of reappearance of these veins at intervals of about 800 meters, vein pattern shows mesh-like arrangement in the mine area. Moreover, bonanza in the veins is controlled by the respective charactristics of each stage's vein structures. Based on interpretation of these geological peculiarities, exploration in the Kushikino mine area since 1968 has resulted discovery of Arakawa No. 3 vein and confirmed the lower parts of Arakawa No. 2 vein.